1. Field of the Invention
The present invention relates to an improvement of a cell supporting member in a solid oxide fuel cell for providing an electric energy through an electrochemical reaction.
2. Description of the Prior Art
The conventional solid oxide fuel cell known in the art comprises a first cell and a second cell provided adjacently to each other and each provided with a slender, annular, porous supporting tube and, provided on the surface thereof, an inner annular electrode, an outer annular electrode and an electrolyte interposed between the inner and outer annular electrodes, and an interconnector passing through the outer annular electrode and the electrolyte to reach a selected partition of the inner electrode, the inner annular electrode of the first cell being connected in series to the outer annular electrode of the second cell through the interconnector of the first cell and a metallic felt disposed between the first cell and the second cell (see Japanese Patent Laid-Open No. 130381/1982).
A supporting member made of alumina or calciastabilized zirconia (CSZ) as disclosed in Japanese Patent Laid-Open No. 33853/1989 is known as the supporting member (corresponding to the supporting tube in the above-described solid oxide fuel cell) of the solid oxide fuel cell.
When the supporting tube made of alumina is used, however, since the coefficient of thermal expansion of the electrolyte (yttria-stabilized zirconia [YSZ]) in the cell part formed on the surface of the supporting tube is different from that of the alumina of the supporting tube, thus cracking occurs in a densely formed electrolyte film during the use of the cell (at a high temperature). This brings about leakage of a fuel (for example, hydrogen) which decreases the difference in the concentration between the outer electrode side and the inner electrode side (in general, the fuel may be fed to any of the inner electrode and the outer electrode) so that the chemical potential difference decreases, which lowers the electromotive force. Further, hydrogen leakage causes reduction of the air electrode constituting the outer electrode or inner electrode. Further, the leaked hydrogen generates heat, which brings about unfavorable cracking in the locally heated supporting tube.
When a supporting tube made of CSZ is used, the above-described problem derived from the difference in the coefficient of thermal expansion between the materials does not occur. In this case, however, when the cell part is formed on the CSZ supporting tube, for example, by means of thermal spraying, the temperature rises particularly on the thermally sprayed surface since CSZ has a small thermal conductivity. This brings about local expansion, causing unfavorable cracking to occur. Further, the CSZ itself is expensive.